Experiments are in progress to obtain monoclonal antibodies directed against immune response-associated (Ia) antigens of the rat. Rats have been immunized against spleen and lymph node cells of other rats bearing incompatible Ia antigen, and spleen and lymph node cells of the immune rats were fused with mouse plasmocytoma cells. Supernatants from the hybrid cell lines have been assayed for complement-dependent cytotoxic antibodies, and positive clones have been identified. The antibody from one of these clones is specific for a B cell antigen determined by an MHC-linked gene.